The usual technique for forming fused biconical taper fibre optic couplers entails disposing respective segments of two or more optical fibres in intimate side-by-side relationship and heating the segments to a temperature sufficient to cause the fibre segments to softer and fuse together. The heated fibres are then longitudinally drawn to cause each of the fused fibres to develop a biconical taper and to thereby form the coupler. A typical disclosure of these steps is to be found in U.S. Pat. No. 4612028 to Abebe.
Known means for effecting heating of the intimately contacted fibre segments include electrical resistance coils, the flames of gas microtorches, and electric arcs. Coils have generally been considered too slow in their heating and cooling rates, and lacking in adequate heat capacity, but flames are liable to cause significant physical damage to the delicate fibres, and also generate undesirable combustion coataminates unless special precautions are taken. For example, U S. Pat. No. 4,439,221 to Smyth et al discloses an improvement on these traditional heating arrangements in which the intimately contacted fibre segments are enclosed in the heating zone within a quartz tube. The fibres are threaded through the tube, which initially acts to protect the fibres and is heated by a flame and is said to thereby act as a furnace to distribute heat to the fibres. After drawing, the tube is retained as a protective packaging sleeve for the coupler.
Similar tubes, typically of quartz or glass and providing both protection and indirect heating, are described in other references. In some cases, the tube is fused into the coupler during heating; in others, it maintains a separate identity and is retained as a packaging sleeve. Australian patent application 46871/85 to Hitachi Ltd, for example, suggests heating the tube with an electric furnace or an oxyhydrogen flame but considers that the furnace is too slow. The reference discloses protective tubes of annular, semi-circular or U-shaped cross-section.
U.S. Pat. No. 4,377,403 to McLandrich discloses a coupler formed from single-mode fibres. The coupler is manufactured by etching the cladding over a given length, longitudinally twisting the fibres (three to ten turns over approximately two centimeters), and fusing the fibres by use of an electric arc. A sheath is placed around the etched, twisted, and fused fibres. A fluid having a refractive index closely matching that of the claddings of the fibres is introduced into the sheath and its ends plugged.
According to U.S. Pat. No. 4,392,712 to Ozeki, a group of optical fibres are placed in side-by-side contact, a quickly solidifying adhesive agent is applied at two positions along the fibres, and the glued fibres are fed into a fusing apparatus. This fusing apparatus comprises a hollow cylindrical heater (a coil of nichrome wire having an elliptical cross-section), clamps for the fibres, and runners driven by a stepping motor. The optical fibres are passed through the heater so that the lengths of the fibres between the two glued positions are located in the heater. The fibres are clamped, and then tightened so as to be straight by runners.
One set of fibre ends is coupled to light sources while the other set of fibre ends is coupled to photoelectric detectors. The heating is continued until the aformentioned portions are fused together. Fusion is measured by the photodetectors as a reduction in the light transmitted through the fibres. At this point, the runners start to move in opposite directions at 1 to 2 mm/sec. to elongate the heated fibre portions to create biconical taper sections. The elongation is continued until the difference in the output powers of the fibres as detected by the detectors is reduced to 0.5 dB.
A microtorch arrangement is disclosed in U.S. Pat. No. 4,591,372 to Bricheno.
It is considered by the present inventors that these prior heating arrangements, which were often developed for benchtop fabrication, are generally not well adapted to efficient, reproducible and reliable manufacture of fibre optic couplers on a mass production basis For example, with flame heating, there is practical difficulty in producing consistent flame characteristics such as temperature profile and temporal and spatial stability. Gas flow buffeting of fibres is also a problem. With an arc, it is extremely difficult to sufficiently control the spatial extent of the arc for coupler manufacture.